The response of soil respiration to insect induced tree mortality: fusing ecophysiological measurements with ecosystem models

人类活动正在导致大气中的二氧化碳上升，因此我们星球的气候正在发生变化。森林对大气中二氧化碳的含量有着巨大的影响，目前北半球的森林储存了每年由人为排放释放的近一半的二氧化碳。森林是否将继续作为人为排放释放的一些二氧化碳的净汇是不确定的，这取决于许多因素，如未来土地利用变化、气候制度和森林干扰率。在过去的十年里，树皮甲虫的爆发已经覆盖了北美大约4700万公顷的森林，造成了广泛的树木死亡。从卫星图像上可以清楚地看到绿叶面积的巨大损失，并对二氧化碳的吸收产生直接影响。随着呼吸作用取代光合作用，该地区的森林很可能成为大气中二氧化碳的净来源。植物碳是土壤微生物呼吸的来源；其中一些碳是较老的腐烂物质，而另一些则来自最近的光合作用。健康森林中每种碳源的相对贡献尚不清楚，而不太清楚的是，在伴随大量腐烂死亡植物材料的重大死亡事件发生后，这些贡献是否会发生变化。通过对美国科罗拉多州山松甲虫爆发的深入研究，本研究将提高我们对森林在大规模扰动后继续吸收人为排放的能力的理解。在过去的五年中，山树皮甲虫逐渐感染了弗雷泽实验林（FEF）的黄松树。2004年，在林场建立了48个森林调查样地，2004 - 2006年对林分结构、碳库和通量进行了详细测量。自这些样地建立以来，大约有30%的样地感染了山松甲虫（MPB），到2010年，这些样地将代表5年的山松甲虫感染时间序列。我建议测量土壤外排、微生物生物量和不稳定碳和老碳。这将使我能够确定由MPB感染引起的土壤外排下降的幅度和动态，同时控制先前估计的林分特性的变化。作为2002年、2003年和2008年在尼沃特岭进行的不同研究工作的一部分，在离地面1.3米的地方，通过去除树干上的韧皮部来杀死一些选定的树木。这个过程（环带）切断了碳水化合物在地下的运输，在一年的时间里，模仿树皮甲虫的影响，杀死了树木。在围篱前和围篱后1-2年，测量了土壤外排、微生物生物量、不稳定碳库和旧碳库。我建议在2010年7月为期两周的时间内，在围地和相关（非围地）参考地重复这些测量。这些测量被用来参数化一个简单的生态系统模型，该模型已被修改，以利用土壤外排、不稳定碳库测量和微生物生物量的估计。2005年至2010年期间FEF的碳交换预测将与直接观测结果进行比较。通过比较模型的不同表示，我将测试碳循环地下成分的不同表示方式。本研究将直接量化疾病暴发和树木死亡对高海拔森林地下碳循环的影响；深入了解鲜为人知的地下碳循环过程；从而改进这些森林生态系统在气候变化情景下的固碳预测。更广泛地说，这项研究符合理解碳-气候关系以及未来气候对生态系统健康和功能的潜在影响的新需求。

Human activities are causing atmospheric carbon dioxide to rise and as a consequence our planet's climate is changing. Forests exert huge influence over the amount of carbon dioxide in the atmosphere, and northern hemisphere forests currently store nearly half of the CO2 released by anthropogenic emissions each year. Whether forests will continue to act as a net sink for some of the CO2 released by anthropogenic emissions is uncertain and depends upon numerous factors such as future land use change, climate regimes and forest disturbance rates. Over the last ten years, an outbreak of bark beetle has covered approximately 47 million hectares of forest in North America, resulting in widespread tree mortality. The huge loss of green leaf area is clearly visible from satellite imagery and has a direct impact on carbon dioxide uptake. It is likely that forests in the region will become a net source of carbon dioxide to the atmosphere as respiration becomes dominant over photosynthesis. Carbon from plants is the source for microbial respiration in soil; some of this carbon is older decaying material but some is from recent photosynthesis. The relative contribution of each carbon source in healthy forests is unclear, and less clear is whether these contributions will change after a significant mortality event accompanied by large quantities of decaying dead plant material. This proposed research will improve our understanding of the capacity of forests to continue to absorb anthropogenic emissions after large scale disturbances though an intensive study of an outbreak of mountain pine beetle in Colorado, USA. Mountain bark beetle has progressively infected Pinus ponderosa trees at Fraser Experiment Forest (FEF) over the past five years. In 2004, 48 forest survey plots were established in FEF where detailed measurements of the stand structure and carbon pools and fluxes were carried out from 2004 through 2006. Since the establishment of these plots approximately 30% have been infected by mountain pine beetle (MPB), and in 2010, these plots will represent a 5 year chronosequence of MPB infection. I propose to measure soil efflux, microbial biomass and labile and older carbon. This will allow me to determine the magnitude and dyanmics of any decline in soil efflux caused by the MPB infection while controlling for variation in stand properties which were estimated previously. As part of different research efforts at Niwot Ridge in 2002, 2003 and 2008, selected trees were killed by removing phloem from the trunk at 1.3m above the ground. This process (girdling) cuts off the transport of carbohydrates below ground and over a period of a year kills the tree, mimicking the effects of bark beetle. Before, and for 1-2 years immediately after girdling, soil efflux, microbial biomass and labile and older carbon pools were measured. I propose to repeat these measurements over a two week period in July 2010 in the girdled plots and the associated (non-girdled) reference plots. These measurements be used to parameterize a simple ecosystem model which has been modified to make use of soil efflux, labile carbon pool measurements and estimates of microbial biomass. Predictions of carbon exchange at FEF for the period 2005 through 2010 will be compared to direct observations. By comparing different representations of the model I will test different ways of representing the below ground component of carbon cycling. This research will directly quantify the effect of disease outbreak and tree mortality on belowground carbon cycling in high elevation forests; provide insight into the poorly understood process of belowground carbon cycling; thus improving projections of carbon sequestration by these forest ecosystems under changing climate scenarios. More broadly, this research fits centrally into the emerging needs for understanding carbon-climate relationships and the potential effects of future climate on ecosystem health and function.